Billionaire Michael Bloomberg has narrowly won a third term as New York mayor in a race far closer than many had expected.

With 99 per cent of results in this morning, the richest man in New York led his Democrat rival William Thompson Jr by 51 per cent to 46 per cent.

Mr Bloomberg, 67, owner of the Bloomberg news agency, used all the financial muscle at his disposal to ensure a third term.

He spent more than $100 million on the most expensive self-financed campaign in US history.

He has been spending at a rate of $15,000 per hour, including blanket television advertisements and hiring a small army of canvassers. Mr Thompson, by contrast, spent $7 million.

A poll the day before the election had put Mr Bloomberg 12 points ahead. In 2005 he won by a 20-point landslide.

Mr Bloomberg, who has an estimated fortune of $16billion, succeeded in having the city's mayoral term limits changed, allowing him to run for a third time.

He had originally supported a ban on three-term mayors but argued that his financial acumen was needed to see the city through the recession.

The change of tactic, and the scale of his spending, are thought to have alienated some voters in an election which had an unexpectedly low turn-out.

The New York Public Interest Research Group, an independent watchdog, said the amount of money spent was "distorting" the democratic process. Mr Thompson said it was "obscene".

"Conventional wisdom says that historically third terms haven't been too successful, but we've spent the past eight years defying conventional wisdom," Mr Bloomberg said after his win, referring to the city's economic resilience following the September 2001 attacks and its success at lowering crime rates.

"We're going to make the next four years the best yet." MrBloomberg has pledged to cut crime further, reduce the city's carbon emissions, expand mass transit, increase city parkland, improve schools, add affordable housing and diversify the local economy.

Republicans have taken victories in two key US elections for governor. In Virginia, Republican Bob McDonnell won by a comfortable margin, while another, Chris Christie, defeated the Democrat governor in New Jersey.

Congressional seats are also up for grabs in New York and California, where wins could boost Republican morale before crucial mid-term ballots next year.

As tough on crime as smoking and transfats

Michael Bloomberg's most memorable achievement in his two terms as mayor is his success at reducing crime in New York to the lowest levels seen in decades.

On taking office, Mr Bloomberg did nothing to hide his liberal inclinations on social issues. He has remained active on gay rights, often speaking to gay groups, and remains pro-abortion and against the death penalty.

In his first two terms, he mostly lived up to his promise of no-nonsense leadership. The precipitous drops in crime rates that Rudolph Giuliani had achieved continued and in fact steepened under his reign.

Mr Bloomberg wrested control of the city's troubled schools from the state and is credited with significant improvement in graduation rates.

More controversially, he made it illegal for New Yorkers to smoke in bars and restaurants. He also banned transfats from their fast food and forced giants like McDonald's to list calorie numbers on all they sold.

He engineered a boom in development and construction that has changed the face of much of the city. It was seen by some to have favoured developers rather than residents - particularly poorer ones.